API 1104 - B.2.4.4.4. - Hardness Testing

Date 03-31-2005 21:16

Hardness is related to residual stress, not to hydrogen cracking. When the usual welding standards (AWS, ASME, API) were first written, there was no way to measure the residual stress. Hardness was then used to make a comparison, especially after stress relieving (PWHT): if the hardness was less than a certain value that experience showed was acceptable, PWHT was OK. If the hardness was higher, the PWHT was to be repeated.
Now it's possible to measure the residual stress in a weld by X ray difraction, and there are portable instruments that allow you to do that on the welded piece at site, i.e., you don't need to get the welded piece to the laboratory.
However, as far as I know, the standards havn't still taken that advance into consideration.
Giovanni S. Crisi
Sao Paulo - Brazil

By JCaprarulo (**)

Date 04-09-2005 11:50

Thanks a lot for your response.
It's thrue about standards are not taking into consideration what you say.
It means : X ray difraction evaluation need to be made for evaluating residual stress in order to approve Welding Procedure with HAZ hardness values more than 350 HV.
I see that method is new, expensive (equipment) and will require an specialist to evaluate results.
Is there any method more simple ?
Thanks in advance.

By ajoy

Date 04-11-2005 05:54

Addressing to the original question regarding the evaluation for the risk of hydrogen assisted cracking, I feel we have to take into consideration all the factors which contribute to HIC.

1. The amount of diffusible hydrogen in the weld. Which means in practical terms, what kind of filler we have used for the welding. Any welding process depositing a weldmetal with diffusible hydrogen above 10 ml/100 grams of weld metal is not a low hydrogen process and thus is susceptible to HIC provided other three conditions are also present.

2. The weldment runs the risk of HIC if the weld and HAZ microstructure is hardenable and thus susceptible to crack.

3. The weldment runs the risk of HIC if the weld is stressed by residual / thermal stresses due to welding and the member is restrained to move.

4. HIC occures at temperatures below 180 deg.c., above which hydrogen diffuses out relatively fast into a larger area and the pressure doesnt get build up.

Now, in order to evaluate the risk of hydrogen cracking, we need to assess all the factors and then conclude if the weld is safe from Hydrogen induced cracking or not. Thus, a single factor,( HAZ hardness of more than 350 HV for example) cannot be taken as the only parameter to evaluate the risk of hydrogen cracking but is certainly one of them.

By G.S.Crisi (****)

Date 04-11-2005 14:41

I don't know in Argentina, but here in Brazil (another Third World, underdeveloped country) you don't need to buy an X-ray difraction equipment, which is expensive indeed and needs a qualified technician to operate it. There are inspection companies that you can call in to do the service for you.
Giovanni S. Crisi